In chemistry, a synthetic element is a chemical element that does not occur naturally on Earth, and can only be created artificially. So far, 24 synthetic elements have been created (those with atomic numbers 95–118). All are unstable, decaying with half-lives ranging from 15.6 million years to a few hundred microseconds.

Seven other elements were first created artificially and thus considered synthetic, but later discovered to exist naturally (in trace quantities) as well; among them plutonium—first synthesized in 1940—the one best known to laypeople, because of its use in atomic bombs and nuclear reactors.

Atomic mass for natural elements is based on weighted average abundance of natural isotopes that occur in Earth's crust and atmosphere. For synthetic elements, the isotope depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore, for synthetic elements the total nucleon count (protons plus neutrons) of the most stable isotope, i.e. the isotope with the longest half-life—is listed in brackets as the atomic mass.

The first element discovered through synthesis was technetium—its discovery being definitely confirmed in 1936. This discovery filled a gap in the periodic table, and the fact that no stable isotopes of technetium exist explains its natural absence on Earth (and the gap). With the longest-lived isotope of technetium, 98Tc, having a 4.2-million-year half-life, no technetium remains from the formation of the Earth. Only minute traces of technetium occur naturally in the Earth's crust—as a spontaneous fission product of uranium-238 or by neutron capture in molybdenum ores—but technetium is present naturally in red giant stars.

The discoveries of mendelevium, nobelium, and lawrencium followed. During the height of the Cold War, teams from the Soviet Union and United States independently discovered rutherfordium and dubnium. The naming and credit for discovery of those elements remained unresolved for many years but eventually shared credit was recognized by IUPAC/IUPAP in 1992. In 1997, IUPAC decided to give dubnium its current name honoring the city of Dubna where the Russian team made their discoveries since American-chosen names had already been used for many existing synthetic elements, while the name rutherfordium (chosen by the American team) was accepted for element 104. Meanwhile, the American team had discovered seaborgium, and the next six elements had been discovered by a German team: bohrium, hassium, meitnerium, darmstadtium, roentgenium, and copernicium. Element 113, nihonium, was discovered by a Japanese team; the last five known elements, flerovium, moscovium, livermorium, tennessine, and oganesson, were discovered by Russian–American collaborations and complete the seventh row of the periodic table.

No elements with an atomic number greater than 99 have any uses outside of scientific research, as they have extremely short half-lives and have not been produced in macroscopic quantities.

All elements with atomic numbers 1 through 94 are naturally occurring at least in trace quantities, but the following elements are often produced through synthesis. Technetium, promethium, astatine, neptunium, and plutonium were discovered through synthesis before being found in nature.